Galatians 4:21-23

Tell me, you who are bent on being under the Law, do you not listen to [what] the Law [really says]? For it is written that Abraham had two sons, one by the slave woman [Hagar] and one by the free woman [Sarah]. [Gen 16:15; 21:2, 9] But the child of the slave woman was born according to the flesh and had an ordinary birth, while the son of the free woman was born in fulfillment of the promise.